1. Field of the Invention
The invention, in general, relates to a method of determining the weight of laundry in the drum of a washing machine and, more particularly, to a method of determining the weight of laundry in a washing drum rotatably mounted within a sudsing container suspended in the housing of a washing machine by springs in its upper region and in its lower region by at least one shock absorber consisting of a cylindrical housing and a frictionally coated piston rod reciprocally movable therein between two abutments with associated spring elements, the drum cooperating with a system for defining the weight of the laundry in the drum with a measuring circuit for determining the lowering of the sudsing container as a function of the weight.
2. The Prior Art
A washing machine including a system for determining the weight of laundry contained in the drum is manufactured and sold under the designation W 377 WPS by the assignee of the instant application. It is also known from German patent specification DE 199 46 245 A1. Washing machines of this type are provided with a displacement sensor arranged parallel to the shock absorber. The sudsing container with the washing drum rotatably mounted therein is connected to the housing by springs. As soon as laundry is placed in the drum the total mass of the sudsing container changes. As a result of the increasing mass the springs are extended or displaced further so that as the amount of the laundry placed into the drum is increased the downward movement of the sudsing container increases as well. The resulting level of the sudsing container is determined by means of the displacement sensor. Accordingly, the displacement sensor releases a signal the difference of which relative to a zero position, i.e. the empty drum, is proportional to the weight of the deposited laundry.
For a spinning operation it is necessary to attenuate or dampen the oscillating amplitude at least during transition through the critical number of rotations (resonance of the spring-mass-system consisting of the springs and the mass of the filled sudsing container). This is accomplished by at least one shock absorber positioned between the sudsing container and the bottom of the housing. Weighing of the laundry takes place while the sudsing container is at rest. At this point in time, the shock absorbers must not generate any forces, as they would have to be overcome by the force of the weight of the laundry before the sudsing container could move downwardly and thereby generate a displacement signal measurable by the displacement sensor. Therefore, the W 377 WPS washing machine uses oil-hydraulic shock absorbers of velocity proportional damping power the damping power of which is very low when they are not moved. Ideally, there would be no attenuation at all. Only when the sudsing container is lowered, velocities arise which generate of counter forces which limit the oscillating amplitude. The disadvantages of the oil-hydraulic shock absorbers reside in their high price, low damping action, acoustic problems and low environmental compatibility because of special requirements relating to the disposal of the oil.
German patent specification DE 49 18 599 A1 discloses a so-called dash-pot damper. It is, in fact, a frictional damper in which the frictional coating is movably mounted on a piston rod between two spring elements, each spring element being supported by an abutment connected to the piston rod. Such an arrangement results in a damping action which is dependent upon amplitude.
From German patent specification DE 100 46 712 A1 it is known to use a dash-pot damper in combination with a displacement sensor. The disadvantage of this measuring system of a dash-pot damper with but one displacement sensor is its undefined initial state. If the drum is empty, it is possible that the frictional coatings are in a position which does not correspond to the zero position of the sudsing container. The suspension springs then exert a force upon the sudsing container which is less than the frictional force of the friction coating. Thus, there is an undefined force which act against the system and which leads to an additional extension of the springs on which the assembly is suspended. The extension of the springs, in turn, is added to the displacement measured by the displacement sensor. Unlike arrangements utilizing a velocity proportional damper (e.g. an oil-hydraulic damper) it is thus not possible to define the weight of the deposited laundry. Hence, the weight of the laundry can only be determined with an initially empty drum which is filled afterwards. However, if the washing machine in its idle state is used as a laundry collector or hamper, it will not be possible subsequently to switch on the washing machine and to obtain a correct reading of the weight of the laundry. Hence, such a machine is of limited utility.
German patent specification DE 101 22 749 A1 discloses a washing machine and, in connection therewith, a method of determining the weight of laundry in which a dash-pot damper is used which is provided with two displacement sensors. Displacement sensors are relatively expensive components, and their added costs take away any economic advantage otherwise to be derived from cost-efficient shock absorbers.